Series connection of two light emitting diodes through semiconductor manufacture process

ABSTRACT

A semiconductor structure with two light emitting diodes in series connection is disclosed. The semiconductor structure comprises two light emitting diodes (LEDs) having the same stack layers and abutting each other but spaced by an isolation trench. The stack layers from a bottom thereof include a thermal conductive substrate, an nonconductive protective layer, a metal adhering layer, a mirror protective layer, a p-type ohmic contact epi-layer, a upper cladding layer, an active layer, and a lower cladding layer. Two p-type ohmic contact metal electrodes for two LEDs are formed on an interface between the mirror protective layer and the ohmic contact epi-layer and buried in the mirror protective layer. The stack layers have first trenches formed therein which exposes the upper cladding layer and electrical connecting channels to connect p-type electrodes. The isolation trench is formed by patterning the exposed upper cladding layer until further exposing the nonconductive protective layer. Two n-type electrodes are formed on the lower cladding layer of two LEDs. A dielectric layer is deposited to fill the isolation trench and covered a sidewall of the first trench so that it can electrically isolate layers of the stack layers of the second LED while a metal connection trace formed thereon to connect the p-type ohmic contact electrode of the first LED and n-type of ohmic electrode of second LED.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device, and moreparticularly to two AlGaInP light emitting diodes in series connectionby semiconductor manufacture process.

2. Description of the Prior Art

The conventional AlGaInP LED has a double hetero-structure (DH), asshown in FIG. 6. The LED stacked sequentially, from a bottom thereof,has an n-type ohmic contact electrode 2, a GaAs substrate 3, an n-type(Al_(x)Ga_(1-x))_(0.5)In_(0.5)P lower cladding layer 4 with an Alcomposition between about 70%-100%, an (AlGa_(1-x))_(0.5)In_(0.5)Pactive layer 5 with an Al composition of 0%-45%, a p-type(Al_(x)Ga_(1-x))_(0.5)In_(0.5)P upper cladding layer 6 with an Alcomposition 70%-100%, a p-type high energy band gap current spreadinglayer 7 such as layers of GaP, GaAsP, AlGaAs or GaInP, and a p-typeohmic contact layer 8 as well as a bonding pad 9.

With the composition alternation of the active layer 5, the wavelengthsof the light emitted are varied from 650 nm: red to 555 nm: green. Adrawback is generally found in the conventional LED, that is: while thelight emitted from the active layer 5 towards the substrate 3 will betotally absorbed by GaAs substrate 3. It is because the GaAs substratehas an energy gap smaller than that of the active layer 5. Therefore,the light generated is absorbed resulted in lower light generatedefficiency for this kind of conventional AlGaInP LED.

To overcome the substrate 3 light absorption problem, severalconventional LED fabrication technologies have been disclosed. However,those conventional technologies still accompany with severaldisadvantages and limitations. For example, Sugawara et al. disclosed amethod published in Appl. Phys. Lett. Vol. 61, 1775-1777 (1992),Sugawara et al. inserted a distributed Bragg reflector (DBR) layer inbetween GaAs substrate and lower cladding layer so as to reflect thoselight emitted toward the GaAs substrate. However, the reflectivity ofDBR layer is usefully only for those light which almost verticallytowards the GaAs substrate. With the decrease of injection angle, thereflectivity is drastically decreased. Consequently, the improvement ofexternal quantum efficiency is limited.

Kish et al. disclosed a wafer-bonded transparent-substrate (TS)(Al_(x)Ga_(1-x))_(0.5)In_(0.5)P/GaP light emitting diode, entitled “Veryhigh efficiency semiconductor wafer-bonded transparent-substrate(Al_(x)Ga_(1-x))_(0.5)In_(0.5)P/GaP” on Appl. Phys. Lett. Vol. 64, No.21, 2839 (1994). The TS AlGaInP LED was fabricated by growing a verythick (about 50 μm) p-type GaP window layer by hydride vapor phaseepitaxy (HVPE) formed on epi-layers light emitting structure.Subsequently, the temporary n-type GaAs substrate is selectively removedusing conventional chemical etching techniques. After removing the GaAssubstrate, the LED epilayer structure is then bonded to an 8-10 milthick n-type GaP substrate.

For the light illuminated concerned, the TS AlGaInP LED exhibits a twofold improvement in light output compared to absorbing substrate (AS)AlGaInP LEDs. However, the fabrication process of TS AlGaInP LED is verycomplicated. Since the bonding process is to make two III-Vsemiconductor wafers directed bond together by heating and pressing fora period of time. Even worse, a non-ohmic contact interface between themis generally found to have high resistance. To manufacture these TSAlGaInP LEDs in high yield and low cost is difficult as a result.

Another conventional technique was proposed by Horng et al., on Appl.Phys. Lett. Vol. 75, No. 20, 3054 (1999) entitled “AlGaInPlight-emitting diodes with mirror substrates fabricated by waferbonding.” Horng et al., reported a mirror-substrate (MS) ofAlGaInP/metal/SiO₂/Si LED fabricated by wafer-fused technology. In LED,AuBe/Au stack layer function as a bonding layer for silicon substrateand epi-layer LED. However, the intensity of the AlGaInP LED is onlyabout 90 mcd under 20 mA injecting current. The light intensity is atleast lower than that of TS AlGaInP LED by 40%. It could not besatisfied.

SUMMARY OF THE INVENTION

A semiconductor structure with two light emitting diodes in seriesconnection is disclosed. The semiconductor structure comprises two lightemitting diodes (LEDs) having the same stack layers and abutting eachother but spaced by an isolation trench. The stack layers from a bottomthereof include a thermal conductive substrate, an nonconductiveprotective layer, a metal adhering layer, a mirror protective layer, ap-type ohmic contact epi-layer,

The two LEDs, respectively, have a first trench formed therein exposedthe p-type ohmic contact epi-layer. An electrical conductive channel isformed in each of the first trench bottom to expose the p-type ohmiccontact metal electrode. Two n-type ohmic contact metal electrodes areformed on the lower cladding layer. A bonding metal layer is then formedto connect the p-type ohmic contact metal electrodes through theelectrical conductive channel and on the n-type ohmic contact metalelectrodes.

To isolate two LEDs, an isolation trench to isolate therebetween isformed at a border of the first trench from the p-type ohmic contactepi-layer to the nonconductive protective layer. A dielectric layer isformed to fill in the isolation trench and extend to one sidewall of thefirst trench which is a boundary between the two LEDS. A conductivemetal trace is then formed on the dielectric layer and extended toconnect the n-type ohmic contact metal electrode of one LED to thep-type ohmic contact metal electrode of the other LED.

Alternatively, the metal adhering layer can be served as one electrodewithout the electrical conductive channel since it had already connectedto the p-type ohmic contact metal electrode. In the situation, thebottom of the first trench is deeper than aforementioned first trenchand the isolation trench is formed from the metal adhering layer to thenonconductive protective layer. The boding metal layer, the dielectriclayer and the conductive trace are the same as first preferredembodiment..

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1A is a schematic cross-sectional view of the light emitting diodehaving a mirror protective layer formed over the first ohmic contactelectrode according to the present invention.

FIG. 1B is a schematic cross-sectional view of the light emitting diodewith an additional channel formed in a mirror protective layer of thenonconductive type according to the present invention.

FIG. 2 is a schematic cross-sectional view of the conductive substrateformed thereover with a nonconductive protective layer and a metaladhesive layer according to the present invention.

FIG. 3A and FIG. 3B are schematic cross-sectional views of a series offabricating process for a light emitting diode having a first trenchexposed the p-type ohmic contact epi-layer and an electric conductivechannel formed thereafter according to the preferred embodiment of thepresent invention.

FIG. 4A and FIG. 4B are schematic cross-sectional views of a series offabricating process for a light emitting diode having a first trenchexposed the metal adhering layer and metal bonding pads formedthereafter according to the preferred embodiment of the presentinvention.

FIG. 5A shows a schematic cross-sectional view of two light emittingdiodes in series connection according to the preferred embodiment of thepresent invention.

FIG. 5B shows an equivalent circuit of the structure shown in FIG. 5A.

FIG. 6 is a schematic cross-sectional view of a conventional lightemitting diode.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention discloses a light emitting diode (LED) structureand a method of two adjacent LEDs in series connection through thesemiconductor manufacture process. The detailed descriptions accompanywith the FIG. 1A to FIG. 5B are as follows.

Since the basis structure for the two LEDs is the same before depictingthe processes about how to connect them. Hence, for illustratingconvenience, only one LED is shown in the FIG. 1 to 4. The mainstructure of two adjacent LEDs in series connection is shown in FIG. 5A.

Referring to FIG. 1A, the cross-sectional view shows an epi-LED stackstructure comprises, from a bottom thereof, an n-type temporary GaAssubstrate 26 an etching stop layer 24, a lower cladding layer 22 formedof n-type (Al_(x)Ga_(1-x))_(0.5)In_(0.5)P, an active layer 20 of undoped(Al_(x)Ga_(1-x))_(0.5)In_(0.5)P, an upper cladding layer 18 of p-type(Al_(x)Ga_(1-x))_(0.5)In_(0.5)P, an ohmic contact epi-layer 16 of p-type(Al_(x)Ga_(1-x))_(0.5)In_(0.5)P. Thereafter, a p-type ohmic contactmetal electrode 28 is formed on the ohmic contact epi-layer 16 byconventional processes. A mirror protective layer 30 is then depositedon the resulted surface.

The material of mirror protective layer 30 can be chosen from conductiveoxide or non-conductive oxide. The conductive oxide, for example, can bea layer of InSnO, In₂O₃, SnO₂, ZnO, or MgO. An example of non-conductiveoxide can be a layer of Al₂O₃, SiO₂, SiNx.

The p-electrode, apart from direct contacts the p-type ohmic contactmetal electrode 28, it can also contact the metal adhering layer 14 inaccordance with the present invention. In case of contacting the metaladhering layer 14, the mirror protective layer 30 formed ofnonconductive oxide should include a plurality of conductive channelstherein to fill with a metal adhering layer 14 thereto electricalconnect the p-type ohmic contact metal electrode 28 to the metalreflector. The result is shown in FIG. 1B.

The p-type ohmic contact epi-layer 16 can be a layer selected from GaP,GaAsP, AlGaAs or GaInP, All of the candidates for serving as the p-typeohmic contact epi-layer 16 require having an energy band gap larger thanthat of the active layer 20 thereby alleviating the light absorption.Moreover, the p-type ohmic contact epi-layer 16 usually must have highcarrier concentrations doped therein so as to form a good ohmic contact.The (Al_(x)Ga_(1-x))_(0.5)In_(0.5)P active layer 20 is with Alcomposition of about x=0 to 0.45. The Al dosage in the upper claddinglayer 18 and lower cladding layer 22 is of about x=0.5 to 1.0. Forsituation of without Al containing, the wavelength of the light emittedfrom Ga_(0.5)In_(0.5)P LED is about 635 nm, which is in range of redvisible light.

As is known by skilled in the art, the ratio of forgoing compound is,for example of the preferred embodiment only, not intended to limit theclaim scope. The invention is also applied to any ratio of thecomposition. Furthermore, the structure of active layer 18 can be asingle hetero-structure (SH), a double hetero-structure (DH), ormultiple quantum wells (MQW). The 0thickness of the upper cladding layer18, active layer 20, and lower cladding layer 22 are respectively,0.5-3.0 μm, 0.5-2.0 μm and 0.5-3.0 μm.

The preferred material of the etching stop layer 24 according to thepresent invention can be any III-V compound semiconductor material if itcan match with that of the GaAs substrate 26 so as to reduce thedislocation density. Another constraint condition for a material to beas a candidate of the etching stop layer 24 is the etching selectivelythereof. The etching stop layer 24 should be with a lower etching ratethan the GaAs substrate 26.

The good candidates of those satisfied above conditions, for examples,InGaP or AlGaAs can be served. The lower cladding layer 22 can also beserved as the etching stop layer 24 since it has a high selectivity toGaAs substrate 26, and thus if the thickness of the lower cladding layer22 is thick enough, the etch stop layer 24 is optional.

Subsequently, a substrate structure as shown in FIG. 2 is prepared. Thestructure comprises a metal adhering layer 14, a nonconductiveprotective layer 12, and a high conductive substrate 10. Examples ofmetal adhering layer 14 are in, Au, Al, and Ag. The high conductivesubstrate 10 can be a semiconductor wafer of silicon, silicon carbide,or GaP or a metal substrate of Au, Al, Cu. The nonconductive protectivelayer 12 can be a layer of Al₂O₃, SiO₂, SiNx, SOG (spin on glass),silicone, BCB (B-staged bisbenzocyclobutene), epoxy, or polyimide.

Subsequently, the light emitting element as those shown in FIG. 1A or 1Bis mounted to the high conductive substrate by bonding the metaladhering layer 14 with the mirror protective layer 30. The processesinclude heating a period of time at a temperature of about 200 to 600°C. incorporate an appropriate pressure.

Thereafter, the opaque n-type GaAs substrate 26 is then removed andstopped at the etching stop layer 24 by an etchant mixture, for example,5H₃PO₄:3H₂O₂:3H₂O or 1NH₄OH:35H₂O₂. If the material of the etching stoplayer 24 is chosen from InGaP or AlGaAs, the layer 24 is preferably tobe removed completely since those materials can still absorb the light.

To make LED with the n electrode and the p electrode on one side, twoetching steps are successively carried out. Referring to FIGS. 3A and3B, the first step is to form a trench to expose the p-type ohmiccontact epi-layer 16. The first patterning step is to removesequentially a portion of the n-type lower cladding layer 22, the activelayer 20 and the p-type upper cladding layer 18 until the p-type ohmiccontact epi-layer 16 is exposed. Afterward, a second pattern step iscarried out to form a slanting channel 31 to expose the p-type ohmiccontact metal electrode 28.

Thereafter, a photoresist pattern (not shown) is coated on all areas.The photoresist pattern (not shown) having an opening exposed a portionof n-type lower cladding layer 22 to define position of n-type ohmiccontact electrode 32. An ohmic contact metal layer 32 is then depositedon all areas including the portion on the n-type lower cladding layer 22and on the photoresist pattern. Afterward, a liftoff process isperformed to remove the ohmic contact metal layer (not shown) on thephotoresist pattern. And then stripping away the photoresist pattern isdone.

Still referring to FIGS. 3A and 3B, a photoresist layer (not shown)having openings over the ohmic contact electrodes 32 and the slantingchannel 31 is coated on all areas. Then a bonding metal layer is thenrefilled the openings and deposited on the photoresist layer. A liftoffprocess and photoresist stripping are then successively followed. Itresults forming bonding pads 34, as is shown in FIG. 3A and FIG. 3B.

The processes of forming two adjacent LEDs in series connection areshown in FIG. 5A. In FIG. 5A, an etching step is further performed toform an isolation trench by chemical etch, or ionic etch the p-typeohmic contact epi-layer 16, mirror protective layer 30, metal adheringlayer 14, and the nonconductive protective layer 12 abutting the bondingmetal layer 34 at the mesa. The isolation trench is to separate the LEDsubstrate into two LED chips with the same structure and having the highconductive substrate 10 in common. An isolation layer formed of Al₂O₃,SiO₂, SiNx, SOG (spin on glass), silicone, BCB (B-stagedbisbenzocyclobutene), epoxy, or polyimide by conventional lithographicdeposition, and liftoff is then formed on the sidewall of the LED chipand refills the isolation trench where the sidewall is abutting theisolation trench. A conductive trace 38 is then formed on the isolationlayer adjoining the p-type ohmic contact metal electrode of the firstchip and the n-type ohmic contact metal electrode of the other chip toimplement the two chips series connection. The result is shown in FIG.5A. FIG. 5 b shows a schematic circuit diagram.

Since the aforementioned LED in first preferred embodiment includesmetal adhering layer 14, which electrically connects to the p-type ohmiccontact metal electrode 28, hence, the trench bottom can be formedalternatively at the metal adhering layer 14 instead of the p-type ohmiccontact epi-layer 16. The second etch step of forming slating trench isthus can be skipped. The results are shown in FIG. 4A for conductivetype mirror protective layer 30 and FIG. 4B for non-conductive mirrorprotective layer 30. The processes of forming metal bonding pads 34 andconductive trace 38 to connect two LED in series are the same as firstpreferred embodiment.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the spirit and scope of the invention.

1. A semiconductor structure of series connection of two light emittingdiodes, comprising: a first light emitting diode having stack layersfrom a bottom thereof including a thermal conductive substrate, annonconductive protective layer, a metal adhering layer, a mirrorprotective layer, an ohmic contact epi-layer, a upper cladding layer, anactive layer, and a lower cladding layer, further, a first ohmic contactmetal electrode formed on an interface between said mirror protectivelayer and said ohmic contact epi-layer and buried in said mirrorprotective layer, a second ohmic contact metal electrode formed on saidlower cladding layer; a second light emitting diode having the samestack layers as said first light emitting diode; still wherein both ofsaid light emitting diodes, respectively, have a first trench formedtherein which has a bottom exposed said ohmic contact epi-layer and saidtrench bottom contains an electrical conductive channel connecting saidfirst ohmic contact metal electrode; an isolation trench formed at aborder of said first trench and with a bottom exposed said nonconductiveprotective layer to isolate said first light emitting diode and saidsecond light emitting diode; a dielectric layer formed to fill in saidisolation trench and extend to a sidewall of said first trench, saidsidewall being a boundary between said first and said second lightemitting diode; and a metal bonding layer formed on said dielectriclayer and extended to connect said second ohmic contact metal electrodeof said first light emitting diode and said second ohmic contact metalelectrode of said second light emitting diode.
 2. The semiconductorstructure according to claim 1, wherein said mirror protective layer isselected one from the group consisting of ITO, In2O3, SnO, ZnO, MgO,Al₂O₃, SiO₂, SiNx, SOG (spin on glass), silicone, BCB (B-stagedbisbenzocyclobutene), epoxy, polyimide, and the combination thereof. 3.The semiconductor structure according to claim 2, further comprisingchannels in said mirror protective layer to provide a connection betweensaid metal adhering layer and said first ohmic contact metal electrodewhile said mirror protective layer is a non-conductive type.
 4. Thesemiconductor structure according to claim 1, wherein said thermalconductive substrate is selected from the group consisting of Al, Au,Cu, Si, and SiC.
 5. The semiconductor structure according to claim 1,wherein said metal adhering layer is selected one from the groupconsisting of In, Au, Ag, and Al and said bonding metal layer isselected one from the group consisting of Au and Al.
 6. A method offorming series connection of two light emitting diodes, said methodcomprising the steps of: providing a temporary substrate, formingsequentially an n-type etch stop layer, an lower cladding layer, anactive layer, an upper cladding layer, and a p-type ohmic contactepi-layer; forming two p-type ohmic contact metal electrodes on saidp-type ohmic contact epi-layer to define a first light emitting diodeand a second light emitting diode; forming a mirror protective layer onsaid p-type ohmic contact epi-layer and covering said two p-type ohmiccontact metal electrodes; providing a thermal Conductive substrate;forming an isolation layer on said thermal conductive substrate; forminga metal adhering layer on said isolation layer; bonding said thermalconductive substrate to said mirror protective layer with said metaladhering layer as an adhering agent; removing said temporary substrateand said n-type etch stop layer; patterning said lowering claddinglayer, said active layer and said upper cladding layer to form a firstand a second trench respectively, for said first light emitting diodeand said second light emitting diode; patterning bottoms of said firstand said second trench to form two connection channels, respectively,connecting said two p-type ohmic contact metal electrodes; patterning abottom of said first trench to form an isolation trench till saidisolation layer is exposed so that said first and said second lightemitting diode are spaced by said isolation trench; forming two n-typeohmic contact metal electrodes, respectively, on said lower claddinglayer of said first and second light emitting diode; forming adielectric layer filling said isolation trench and on a sidewall of saidfirst trench, said sidewall being a boundary between said first and saidsecond light emitting diode; and forming bonding metal pads on said twon-type ohmic contact metal electrodes and said two connection channels;forming a conductive trace on said dielectric layer and extended toconnect said p-type ohmic contact metal electrode of said first lightemitting diode to said n-type second ohmic contact metal electrode ofsaid second light emitting diode.
 7. The method according to claim 6,wherein said step of forming a first and a second trench furthercomprises patterning more until said metal adhering layer is exposed. 8.The method according to claim 6 wherein said mirror protective layer isselected one from the group consisting of ITO, In₂O₃, SnO, ZnO, MgO,Al₂O₃, SiO₂, SiNx, SOG (spin on glass), silicone, BCB (B-stagedbisbenzocyclobutene), epoxy, polyimide, and the combination thereof. 9.The method according to claim 6, further comprising forming channels insaid mirror protective layer before the step of bonding said thermalconductive substrate to said mirror protective layer to provide aconnection between said metal adhering layer and said p-type ohmiccontact metal electrode while said mirror protective layer is anon-conductive type.
 10. The method according to claim 6, wherein saidthermal conductive substrate is selected from the group consisting ofAl, Au, Cu, Si and SiC.
 11. The method according to claim 6, whereinsaid metal adhering layer is selected one from the group consisting ofIn, Au, Ag, and Al and said bonding metal layer is selected one from thegroup consisting of Au and Al.
 12. A method of forming series connectionof two light emitting diodes, said method comprising the steps of:providing a temporary substrate, forming sequentially an n-type etchstop layer, an lower cladding layer, an active layer, an upper claddinglayer, and a p-type ohmic contact epi-layer; forming two p-type ohmiccontact metal electrodes on said p-type ohmic contact epi-layer todefine a first light emitting diode and a second light emitting diode;forming a mirror protective layer on said p-type ohmic contact epi-layerand covering said two p-type ohmic contact metal electrodes; providing athermal conductive substrate; forming an isolation layer on said thermalconductive substrate; forming a metal adhering layer on said isolationlayer; bonding said thermal conductive substrate to said mirrorprotective layer with said metal adhering layer as an adhering agent;removing said temporary substrate and said n-type etch stop layer;patterning said lowering cladding layer, said active layer, said uppercladding layer, said p-type ohmic contact epi-layer, and said mirrorprotective layer to expose said metal adhering layer so as to form afirst and a second trench respectively, for said first light emittingdiode and said second light emitting diode; patterning a bottom of saidfirst trench to form an isolation trench till said isolation layer isexposed so that said first and said second light emitting diode arespaced by said isolation trench; forming two n-type ohmic contact metalelectrodes, respectively, on said lower cladding layer of said first andsecond light emitting diode; forming a dielectric layer filling saidisolation trench and on a sidewall of said first trench, said sidewallbeing a boundary between said first and said second light emittingdiode; and forming bonding metal pads on said two n-type ohmic contactmetal electrodes and on said exposed metal adhering layer; forming aconductive trace on said dielectric layer and extended to connect saidp-type ohmic contact metal electrode of said first light emitting diodeto said n-type ohmic contact metal electrode of said second lightemitting diode.
 13. The method according to claim 12 wherein said mirrorprotective layer is selected one from the group consisting of ITO,In₂O₃, SnO, ZnO, MgO, Al₂O₃, SiO₂, SiNx, SOG (spin on glass), silicone,BCB (B-staged bisbenzocyclobutene), epoxy, polyimide, and thecombination thereof.
 14. The method ,according to claim 13, furthercomprising forming channels in said mirror protective layer before thestep of bonding said thermal conductive substrate to said mirrorprotective layer to provide a connection between said metal adheringlayer and said p-type ohmic contact metal electrode while said mirrorprotective layer is a non-conductive type.
 15. The method according toclaim 12, wherein said thermal conductive substrate is selected from thegroup consisting of Al, Au, Cu, Si and SiC.
 16. The method according toclaim 12, wherein said metal adhering layer is selected one from thegroup consisting of In, Au, Ag, and Al and said bonding metal layer isselected one from the group consisting of Au and Al.